Tidal gullies in youngest peat layer of Groningen

Where the top layer of clayey substance was thin and the underlying peat layer thick, the shrinkage of the latter had been such that the clay-filled gullies appeared as ridges of about ½ to 1 m high in the field. Farmers sometimes call these ridges ,,natural dykes". In some places they have dug up parts of these ancient depots of good clay and...